FIG. 1 depicts a wireless spread spectrum TDD communication system 100. The system 100 has a plurality of base stations 301-307. Each base station 30 communicates with a plurality of wireless transmit/receive units (WTRUs) in its operating area. For example, base station 301 communicates with WTRUs 321-323. Communications transmitted from a base station 30 to a WTRU 32 are referred to as downlink communications, and communications transmitted from a WTRU 32 to a base station 30 are referred to as uplink communications.
In a Universal Mobile Telecommunications System (UMTS) as specified by the Third Generation Partnership Project (3GPP), base stations are called Node Bs, subscriber stations are called wireless transmit/receive units (WTRUs) and the wireless CDMA (Code Division Multiple Access) interface between the Node Bs and UEs is known as the Uu interface.
Node Bs are typically capable of conducting wireless concurrent communications with a plurality of subscriber stations, generically known as WTRUs (i.e., User Equipments (UEs)), which include mobile units.
In addition to be able to communicate over different bands of frequency spectra, spread spectrum TDD systems can carry multiple communications over the same band of frequency spectra. Individual communication signals are distinguishable by their respective chip code sequences (codes). FIG. 2 shows two frames 34A, 34B of a TDD signal 200. Each frame 34A, 34B is divided into 15 time slots 361-3615. In such systems, a communication is sent multiplexed in selected time slots 361-3615 using codes. Accordingly, one repeating frame 34 is capable of carrying multiple communications distinguished by both time slot and codes. The combination of a single code in a single time slot is referred to as a resource unit. One or multiple resource units are assigned to a communication based on the bandwidth required to support the communication.
Most TDD systems adaptively control transmission power levels allowing many base stations and WTRUs to share the same time slots within a radio frequency spectrum. For example, when WTRU 321 is receiving a specific communication from base station 301, all the other WTRUs and base stations communicating using the same time slots and spectrum will generate interference on the downlink communication of base station 301.
One way for base station 301 to ensure delivery of information is by increasing the transmission power level. However, this will degrade the signal quality of all other communications within that time slot and radio frequency spectrum by generating interference. To avoid increasing power of the base station 301, all the other WTRUs and base stations within range may be instructed to decrease their transmit power. However, reducing the transmission power level of these base stations and WTRUs too far, will result in undesirable signal to noise ratios (SNRs) and high bit error rates (BERs) at the receivers.
To maintain both the quality of communications while controlling the transmission power levels, a transmission power control scheme is used. This is especially important on uplink situations where a near-far problem may occur. The near-far problem occurs when a base station (BS) receives a much stronger signal from a WTRU nearby than from another WTRU far away. Since all users share the same frequency band, the near WTRU would drown out the far WTRU. In addition, power control also prolongs battery life while it reduces interference.
One approach using transmission power control in a code division multiple access (CDMA) communication system is described in U.S. Pat. No. 5,056,109 (Gilhousen et al.). A transmitter sends a communication to a particular receiver. Upon reception, the received signal power is measured. The received signal power is compared to a desired received signal power. Based on the comparison, a control bit is sent to the transmitter either increasing or decreasing transmission power by a fixed amount. Since the receiver sends a control signal to the transmitter to control the transmitter's power level, such power control techniques are commonly referred to as closed loop.
Under certain conditions, the performance of a closed loop system will degrade. For example, if communications sent between a WTRU and a base station which are in a highly dynamic environment, such as due to the WTRU moving, such systems may not be able to adapt fast enough to compensate for the changes. The update rate of closed loop power control in TDD is typically 100 cycles per second which is not sufficient to compensate for fast fading channels. For example, a WTRU traveling at 100 kilometers per hour (62 miles per hour) may travel 278 centimeters between updates. In addition, if the WTRU were operating at 881.52 Mhz, the distance traveled between updates would be approximately 294 degrees of a wavelength and may place the WTRU into a deeper null.
Outer loop and weighted open loop power controls are other methods for transmission power control. Outer loop power control utilizes an error detection device to look at the soft symbols and errors produced in a data estimation device. A processor analyzes the detected errors and determines an error rate for the received communication. Based on the averaged error rate, a processor determines a desired target error rate for the communication. The processor determines an amount, if any, the power level needs to be changed at the transmitting station to achieve the desired error rate. An adjustment is subsequently sent to the transmitting station using a dedicated or a reference channel.
To compound the fading problem, a WTRU employing a time slot which is located temporally distant from the reference beacon time slot may reposition itself during this time interval and may move into a deeper null resulting in a deeper fade. For example, a WTRU using time slots in the middle of a standard frame having the fixed length of 10 ms will allow the WTRU to travel an additional one-half wavelength at 108 km/hr. In addition to moving in and out of nulls, the WTRU by its movement introduces Doppler fading into the mix. These affects are more pronounced in systems which have less frequent power control adjustments.
Therefore, compensating for deep fading and Doppler frequency and phase shifting are important to power control gain. In addition to fading and Doppler, the time slot separation between the reference beacon and the WTRU's time slot also has an impact on efficient power control gain. This is especially pronounced in the TDD mode of operation. Consequently, it is desirable to have a method to choose slot locations to optimize uplink power control gain with fading channels while taking into account Doppler effects.